Weldable and wear resistant magnetic ferritic malleable iron and method

ABSTRACT

Weldable and magnetic ferritic malleable iron castings having an improved abrasion resistant wear layer, and the method of forming such castings are described. The casing includes a body portion of full ferritic malleable iron which is covered with several zoned layers, viz., a pearlitic type matrix of broken spheroidized martensite which is partially depleted of carbon nodules, a decarburized wear layer of finely dispersed well distributed spheroidized carbides in a ferritic matrix which provides a good weld zone and an outer intergranular oxidation layer. In machining castings to be used for abrasion resistance the outer two layers and a portion of the wear layer are removed to expose the abrasion resistant wear layer. In the case of a casting to be welded, the outer layer and a portion of the weldable layer are removed. By controlling the structure of the casting between the fully ferritic body and the wear layer, high magnetic permeability and low magnetic remanance are obtained while providing a weldable casting. In forming such castings, white iron of malleable composition is annealed in a decarburizing atmosphere in which the ratio of carbon monoxide to carbon dioxide is closely controlled. The castings are heated to between 1800* F. and 1975* F. for a period of time sufficient to decompose the cementite followed by cooling to 1400* F. to 1650* F., and quenching or air colling, and tempering. Various details of the annealing and tempering cycles are described. rReference is made to U.S. application Ser. No. 605,955, filed Dec. 30, 1966, now U.S. Pat. No. 3,463,675 and assigned to the same assignee.

United States Patent [72] Inventors Oral K. llunsaker; Bruce R. Shue, both of Dayton, Ohio [21] Appl. No. 728,807 [22] Filed May 13, 1968 [45] Patented Sept. 21, 1971 [73] Assignee The Dayton Malleable Iron Company Dayton, Ohio [54] WELDABLE AND WEAR RESISTANT MAGNETIC FERRITIC MALLEABLE IRON AND METHOD 12 Claims, No Drawings [52] U.S. CI 148/16, 148/35, 148/39, 148/138, 148/139, 148/140 [51] Int. Cl C2ld 5/10, C22c 37/02, C22c 37/04 [50] Field otSearch 148/16, 138,139,140,14l

[56] References Cited UNITED STATES PATENTS 2,557,379 6/1951 Hancock et al 148/16 2,796,373 6/1957 Berg 148/139 3,055,779 9/1962 Chu et a1. 148/16 FOREIGN PATENTS 118,157 111945 Australia 148/16 134,195 9/1949 Australia..... 148/16 585,921 2/1947 Great Britain.. 148/ 16 654,537 6/1951 Great Britain.. 148/16 35M? 9 0 9 99151991111: 1:18.0 9

897,159 5/1962 GreatBritain Primary Examiner-L. Dewayne Rutledge Assistant Examiner-W. W. Stallard Attorney-Marechal, Biebel, French & Bugg ABSTRACT: Weldable and magnetic ferritic malleable iron castings having an improved abrasion resistant wear layer, and the method of forming such castings are described. The casing includes a body portion of full ferritic malleable iron which is covered with several zoned layers, viz., a pearlitic type matrix of broken spheroidized martensite which is partially depleted of carbon nodules, a decarburized wear layer of finely dispersed well distributed spheroidized carbides in a ferritic matrix which provides a good weld zone and an outer intergranular oxidation layer. In machining castings to be used for abrasion resistance the outer two layers and a portion of the wear layer are removed to expose the abrasion resistant wear layer. In the case of a casting to be welded, the outer layer and a portion of the weldable layer are .removed. By controlling the structure of the casting between the fully ferritic body and the wear layer, high magnetic permeability and low magnetic remanance are obtained while providing a weldable casting. ln forming such castings, white iron of malleable composition is annealed in a decarburizing atmosphere in which the ratio of carbon monoxide to carbon dioxide is closely controlled. The

, castings are heated to between 1800" F. and 1975 F. for a period of time sufficient to decompose the cementite followed by cooling to 1400 F. to 1650 F., and quenching or air coiling, and tempering. Various details of the annealing and tempering cycles are described.

WELDABLE AND WEAR RESISTANT MAGNETIC FERRI'IIC MALLEABLE IRON AND METHOD Reference is made to U.S. application Ser. No. 605,955, filed Dec. 30, 1966, now U.S. Pat. No. 3,463,675 and assigned to the same assignee.

BACKGROUND OF THE INVENTION This invention relates to a weldable magnetic ferritic malleable iron composition having an abrasion resistant wear layer, and to a method for producing the same.

Malleable iron includes two principal types, standard or ferritic malleable which includes ferrite in which is interspersed nodules of free carbon, and pearlitic malleable in which some of the carbon is present in combined form. The production of malleable iron is a direct process in which scrap, foundry returns including sprue (gates and feeders from previous heats) and the like are the raw materials. Melting is carried out in a cupola, air, are, or induction furnace or in combinations, for example, a duplexing system. Metallurgical inspection is closely controlled during melting with suitable additions as needed to provide the proper chemical composition of the white iron. Following the melting operation, the white iron castings are poured and heat treated for malleabilization, the heat treatment or annealing being variable depending upon whether ferritic or pearlitic malleable iron is the final product.

As cast white iron of malleable composition will solidify with the carbon which is present in the material being in the form of cementite or iron carbide, and when at room tempera ture, will consist of rather large carbides and pearlite, that is, alternate layers of ferrite and cementite. In the malleabilization procedure the combined carbon through the casting is converted into elemental carbon, that is, graphite or temper carbon and ferrite. In first-stage malleabilization or graphization, the white iron castings are heated through the eutectoid range to transform the pearlite into austenite in which carbon from the cementite diffuses into the iron to form a solid solution of carbon and gamma iron.

The first-stage graphitization includes several processes which are carried out simultaneously including solution of the cementite at its interface with austenite, dissolution or diassociation of cementite into iron and carbon, migration of carbon through the austenite or diffusion of matrix atoms away from the nuclei from which the temper carbon grows, and precipitation of graphite. After first-stage graphitization, the structure of the casting consists of graphite, also referred to as temper carbon nodules, which are distributed through the austenite matrix, the latter being a solid solution of gamma iron saturated with an amount of carbon which is dependent upon the particular temperature of the first-stage malleabilization procedure. Usually the first-stage malleabilization is carried out at a temperature of between 1600 F. and 1800" F., and with the exception of a small cross-sectional surface skin, casting is of the same grain structure throughout.

The second stage consists of reducing the temperature of the iron into the eutectoid range wherein the iron exists in three phases, austenite-ferrite-cementite, or austenite-ferritegraphite. The first phase is considered metastable while the second is considered a stable phase. At a temperature slightly below the eutectoid range, any pearlite in the iron will graphitize. By slow passage through the eutectoid range, the iron is fully graphitized or malleabilized and no further structural change takes place at the lower temperatures. The product of full malleabilization is ferritic malleable iron which is substantially free of pearlite structure.

The objective in the formation of pearlitic malleable is to treat the product of the first-stage graphitization in such a manner that the eutectoidal carbides and low-temperature transformation products are purposely retained. The present invention deals with a zoned basically ferritic type malleable iron in which one zone is essentially a pearlite matrix.

SUMMARY OF THE INVENTION The ferritic malleable of the present invention differs from conventional ferritic malleable in that it is composed of layers of predetermined thickness and of different microstructure whereas conventional ferritic usually possesses essentially the same microstructure throughout. The malleable of the present invention has an abrasion resistant quality similar to l020 and 1030 carbon steels with better machinability, and it is easily cast. A further characteristic of the metal of this invention is the provision of a wear surface including spheroidized carbides as opposed to graphitic carbon usually present in ferritic malleable. The free carbon of the ferrite structure permits slip and then abrasive wear, while the spheroidized carbides, especially finely dispersed well distributed spheroidized carbide structured layer of this invention markedly increases abrasion resistance while retaining magnetic penneability and retaining the machinability usually associated with malleable iron. Further, the presence of a layer partially depleted of carbon nodules in a basically ferritic matrix provides a weldable castmg.

It is well known that pure iron, that is, iron free of inclusions exhibits excellent magnetic properties, i..e., high maximum permeability and saturation with negligible coercivity and hysteresis. The presence of certain inclusions has a detrimental effect which varies depending on its type. For example, fine spheroidite is less objectionable than coarse, which is less of a problem than lamellar pearlite, while tempered martensite and flake graphite seriously affect the magnetic character of iron. Of all these inclusions the fine spheroidite, is the least objectionable from the standpoint of its adverse affect on magnetism, and in accordance with this invention, offers advantages as an abrasion resistant weldable matrix.

The structural, layered basically ferrite malleable iron of this invention possesses sufficiently unique characteristics to be useful in magnetic clutch assemblies, e.g., magnetic clutches for automotive refrigeration compressors used in air conditioners. In addition, the metal is easily machined, stamped, and the like, and readily weldable.

Accordingly, it is a primary object of the present invention to provide an improved ferritic malleable iron having an improved wear and abrasion resistant surface while having high magnetic permeability and low magnetic remanance.

Another object of the present invention is the provision of a process of producing ferritic malleable castings of the type described.

Another object of the present invention is the provision of a process of producing ferritic malleable castings which after removal of the outer layer expose a weldable layer.

Another object of this invention is the provision of a weldable ferritic malleable casting having the magnetic properties described and including a body of fully ferritic structure covered with zoned layers, one of the layers being a wear resistant layer of spheroidized carbides in a ferrite matrix and being covered by an outer layer area at least a portion of which is removed during machining to expose the wear resistant layer.

Other objects and advantages of the invention will be apparent from the following description and the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In accordance with the present invention, white iron of malleable composition is poured to form castings, and heat treated or annealed to form a casting in which well defined zoned strata are formed. The melting of the base iron may be by any of the usual procedures, i.e., cupola, air, are or induction furnaces, or a combination thereof in a duplexing" system. Metallurgical inspection is controlled during melting with suitable additions as needed to provide proper chemical composition of the white iron.

The composition of the white iron in accordance with the present invention is as follows:

Carbon 2% to 3% Silicon 0.90% to 2.00% Phosphorus 018% maximum Chromium 0.08% maximum Boron 0.002% to 0.0035% Copper 0.20% maximum Antimony 0.004% maximum Tin 0.02% maximum Sulfur 0.18% maximum Tellurium 0.009% to 0.003% Bismuth 0.009% to 0.02%

Manganese is present in an amount twice that of sulfur with an excess of-between 0.10 percent minimum and 0.20 percent maximum, all percentages being by weight. The balance is iron with small amounts of the ordinary impurities. The controlled amounts of tellurium and bismuth may be obtained using the procedure described in application Ser. No. 605,955 previously identified.

After the castings have been poured, they are annealed under controlled conditions with respect to temperature and atmosphere so that proper metallurgical structure is obtained in the various zones, particularly the wear layer and the weldable layer. The annealing operation in the presence of a controlled atmosphere operates to break down the primary cementite in the as cast white iron and to decarburize the surface portion of the casting to a predetermined depth depending on the gas atmosphere composition and composition of the castings. Accordingly, the temperature of the anneal and the composition of the gas are controlled to effect transformation of the primary cementite and diffusion of carbon from the interior portion of the casting to the surface while at the same time maintaining scaling to a minimum, i.e., the outer surface of the casting has a relatively thin oxide edge or intergranular oxidation layer.

in the first-stage annealing operation, that is, the decomposition of the cementite, atmosphere control is essential in producing the malleable iron of the present invention. Thus, a mixture of carbon monoxide and carbon dioxide in the ratio of L5 to l to 3.5 to l by volume is used and forms the effective decarburizing atmosphere. By maintaining hydrogen gas present in an amount of between percent and 3percent volume of the decarburizing atmosphere, scale formation is substantially eliminated. The remainder of the atmosphere is nitrogen gas.

The temperature of the castings is raised to between l,800 F. to l,975 F. in the controlled atmosphere, and the castings held at that temperature for 24 to 96 hours followed by cooling to 1,400 F. to l,650 F., again in the controlled atmosphere. The castings are held in the range of 1,400 F. to l,650 F. long enough to achieve a stable temperature in that range.

Thereafter, the castings are cooled preferably by an oilquenching operation which provides a matrix having a high combined carbon structure, i.e., a mixture of martensite and other upper transformation products. The details of the oilquenching operation are described in U.S. Pat. No. 3,365,335, issued Jan. 23, 1968, and assigned to the same assignee. As an alternate procedure, the castings may be cooled in still or forced air followed by reheating to l,400 F. to 1,650 F. and oil quenched. The still air cooling provides a low combined carbon, i.e., pearlite and some ferritic grain boundry, while the forced air cooling increases the combined carbon to provide a pearlite structure.

After annealing, the castings are tempered or drawn preferably by reheating to l,370 F. to 1,430 F., and holding the castings at that temperature for a period of one hour for each inch of casting cross section. The castings are then cooled slowly at a rate of 10 F. to 25 F. per hour to a temperature of l,320 F. to 1280 F., and then cooled in still air. An alternate procedure is to temper as above described, but in cooling, hold the castings isothermally at a temperature of between 1,300 F. to 1,320 F. to 2 to 4 hours.

Still another alternative in the tempering operation is to reheat to l,275 F. to l,350 F., and hold the castings at that temperature for 416 hours, followed by still air cooling.

The resultant castings exhibit physical properties the same as or better than grade 32510 malleable iron but differ in important respects. The singular most important difference is the structured strata in the casting, and particularly those strata which form the weld layer and the wear layer, that is, the inner layer in the casting which possesses the abrasion resistant characteristics not found in standard malleable. in addition to the wear layer, the casting possesses high magnetic permeability and low magnetic remanance.

The castings in accordance with the present invention have an outer exposed surface layer which may be described as an oxidation edge or an intergranular oxidation layer which is relatively free of scale. The dimension of this layer varies from 0.007 to 0.010 inch, depending upon the specific conditions of anneal and metal composition. Beneath the intergranular oxidation layer are decarburized layers, the outer of which is basically a ferrite structure having scattered carbon nodules in it, e.g., 2-40 nodules/sq. mm., providing a good weldable microstructure for this type metal. The second decarburized layer, which is the wear layer behind the outer decarburized layer contains a relatively small amount of carbon nodules, l040 nodules per/sq. mm., and scattered finely dispersed well distributed spheroidized carbides in a basically ferritic matrix. The outer decarburized layer starts at about 0.010 inch from the outer surface and may extend as deep as 0.045 inch, and sometimes as deep as 0.075 inch.

The wear layer, or inner decarburized layer starts at about 0.045 inch and extends back to about 0.250 inch. The body of the casting is a fully ferritic matrix free of pearlite, and between the body and the wear layer is a pearlite matrix which starts at about 0.100 inch from the surface and extends to about 0.300 inch from the surface.

The above dimensions are ranges at which the various layers are found, the layers being easily detectable by the sharp difference in microstructure.

The location and dimensions of the various layers have been preselected because of the common procedures used in machining malleable castings. Usually, a machining operation takes between 0.045 and 0.090 inch off the machined portion of the casting, it being understood that not all castings have all surfaces thereof machined. Normally, it is only the functional surface that is machined. Accordingly, during machining of the castings of the present invention, the depth of cut is coordinated with the depth of the wear or weld layer so that in normal machining of the functional areas, the cut will expose the weld or wear layer, that is, remove the intergranular oxidation layer, all or a portion of the outer decarburized layer and a portion of the wear layer.

In the case of a casting to be used in abrasive-type operations, it is preferred that between 0.030 and 0.090 inch of the wear layer be retained, with the fully ferritic body starting at about 0.030-0.040 inch behind the wear layer. The wear layer is relatively easy to machine, and possesses the spheroidized carbide structure for abrasion resistance not obtainable with ferritic or pearlitic malleable. Additionally, since the spheroids are small, they do not represent inclusions which seriously adversely affect the magnetic characteristics of the casting. For example, when used as a magnetic clutch for automotive refrigeration compressors, the clutch will engage in milliseconds, i.e., with a shaft turning at 6,000 rpm, the clutch will engage within three turns. More important, however, is the fact that the clutch will release within three turns of a shaft rotating at 6,000 rpm. thus indicating the low magnetic remanance. In the case of a refrigeration compressor which provides considerable mass, the abrasive wear during engagement of the clutch is quite high, and the carbide containing and low carbon nodule wear surface of the present invention operates quite satisfactorily as a clutch face.

in addition to the magnetic characteristic, wear capability and weldable nature of the metal, the material of the present invention is easily machined by a punching operation. whereas pearlitic or dense pearlitic malleable is not as easily punched.

In a typical operation, a base iron of malleable composition was annealed and tempered in accordance with the present invention. The composition of the base iron was as follows:

Carbon 2.65

Silicon 1.55

Sulfur 0.12 Manganese 0.36 Phosphorus 0.04 Chromium 0.05

the balance being iron and the other elements in the ranges previously specified. The castings were annealed at a temperature of between 1,800 F. to l,975 F., thereafter cooled to 1,400" F.l,650 F. and oil quenched followed by reheating to l,370 F.-1,430 F., held at that temperature for 1 hour per inch of casting section, and cooled at the rate of 10 to 25 per hour to a temperature between 1,280 F. and 1,320 F. followed by still air cooling, the first stage being completed in 48 hours. The atmosphere during the first stage anneal consisted of carbon dioxide 2.1 percent, carbon monoxide 7.6 percent and hydrogen gas 21 percent, all by volume, with nitrogen gas making up the balance. The resultant casting showed the oxide edge to be a depth of between 0.007 and 0.010 inch, with the ferritic matrix and scattered nodule matrix extending to a depth of 0.035 inch, the wear layer extending to a depth of 0.071 inch and the pearlitic type matrix extending to a depth of 0.107 inch with the ferritic structure forming the body of the casting.

This particular type of ferritic malleable iron is quite versatile, particularly because of its weldability, and thus can be used as a replacement for steel forgings and castings, and is easily welded.

While the articles and method herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise articles and method, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

What is claimed is:

l. A ferritic malleable iron casting having high magnetic permeability and low magnetic remanance and a wear resistant surface comprising a body portion of fully ferritic malleable iron, said body portion being covered with multiple zoned superimposed layers, said ferritic malleable body portion being covered with a partially decarburized layer of broken spheroidized martensite in a pearlitic malleable matrix, said partially decarburized layer being covered with a decarburized weld layer of spheroidized carbides in a ferritic matrix, and said casting containing the following:

Carbon 2% to 3% Silicon 0.90% to 2.00% Phosphorus 018% maximum Chromium 0.08% maximum Boron 0.0020% to 0.00357 Copper 0.20maximum Antimony 0.004% maximum Tin 0.20% maximum Sulfur 0.18% maximum and manganese in an amount of twice that ofsulfur with an excess of 0.20 percent maximum, and 0.10 percent minimum, tellurium and bismuth being present between 0.0009 percent to 0.0003 percent to 0.02 percent, respectively, and the balance being iron plus small amounts of ordinary impurities.

2. A weldable ferritic malleable iron casting as set forth in claim 1 wherein said wear layer is covered with a decarburized weldable layer of scattered carbon nodules in a ferritic mixture.

3. A weldable ferritic malleable iron casting as set forth in claim 2 wherein said decarburized weldable layer is covered by a decarburized intergranular oxidation layer which forms the outside surface layer of said casting.

4. A weldable ferritic malleable iron casting as set forth in claim 3 wherein said decarburized layers extend below the surface of said casting a distance of between 0.010 and 0.250

inches.

5. A weldable ferritic malleable iron casting as set forth in claim 2 wherein said wear layer extends below said decarburized layer of scattered carbon nodules in a ferritic matrix a distance of between 0.045 to 0.250 inches 6. A weldable ferritic malleable iron casting as set forth in claim 1 wherein said decarburized wear layer constitutes the outside surface layer of at least a portion of said casting and has a cross-sectional dimension of between 0.030 and 0.090 inches.

7. A weldable ferritic malleable iron casting as set forth in claim 1 wherein said phosphorus is present in an amount not greater than 0.08 percent.

8. The method of forming weldable ferritic malleable iron castings having high magnetic permeability and low magnetic remanance and wherein the castings include a body portion of fully ferritic malleable iron covered by a layer of partially decarburized spheroidized martensite in a pearlitic malleable matrix which in turn is covered by a decarburized wear layer of spheroidized carbides in a ferritic matrix, said wear layer being covered by a decarburized weldable layer of scattered carbon nodules in a ferritic matrix and wherein said last named layer is covered by an intergranular oxidation layer forming the outside surface layer of the casting, the method comprising the steps of annealing said castings at a temperature of between l,800 F. to 1,975 F. in a decarburizing atmosphere containing carbon monoxide, carbon dioxide and hydrogen gas as the active ingredients, said atmosphere being relatively free of oxygen gas, the ratio of carbon monoxide to carbon dioxide being in the range of 1.5 to l to 3.5 to l by volume, said hydrogen gas being present in an amount of between 15 percent to 30 percent by volume of said decarburizing atmosphere, said annealing being conducted for a period of time sufficient to convert substantially all of the carbides present in said body portion to carbon nodules dispersed in a ferritic matrix, said decarburizing atmosphere being operative to reduce substantially the amount of carbon nodules present in at least said decarburized layers, cooling said castings to a temperature of between 1,400 F. and 1,650 F., and tempering to form spheroidized carbides in a ferritic matrix in said decarburized wear layer.

9. The method as set forth in claim 8 in which carbon dioxide is present in an amount of 2.1 percent by volume, carbon monoxide is present in an amount of 7.6 percent by volume, hydrogen is present in an amount of 21 percent by volume and nitrogen constitutes the remainder of the decarburizing atmosphere.

10. The method as set forth in claim 8 wherein said castings are oil quenched following cooling between l,400 F. to 1,650 F., and said tempering operation includes reheating said castings to between l,370 F. and l,430 F., maintaining said castings at said temperature for a period of 1 hour for each inch of cross section casting thickness, cooling said castings at a rate of l0to 25per hour to a temperature of between 1,320 F. and l,280 F., and air cooling said castings to ambient temperature.

11. The method as set forth in claim 8 wherein said castings are air cooled following cooling to between 1,400 F. and l,650 F., thereafter reheating said castings to a temperature of between l,400 F. and 1,650 F. followed by oil quenching of said castings, and said tempering operation including reheating said castings to a temperature of between l,275 F. and 1,350 F. and holding said castings at said temperature for a period of between 4 and 16 hours followed by cooling in air to ambient temperature.

12. The method as set forth in claim 10 wherein said step of cooling to 1,320 F. to 1,280 F. includes the step of maintaining said castings at a temperature of between l,300 F. to 1,320 F. for a period of from 2 to 4 hours.

mg? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, D d September 2].,

Oral K. Hunsaker and Bruce R. Shue Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

F- In the Abstract, line 24 "colling" should be ecooling-m .1

Column 3, line l0,"0.009%" should be -0.0009%w-; line 11, "0.009%" should be -0.0009%-; line 41, "3% should be ---30%-'-. Column 4 line 4 "strata" should be "-stratas (first and second occurrence); line 18, "2" should be -ZO -w Column 5, line 55, "0. 20% should be --.02%--; line 62 "0.0003%" should be 0.003%; line 63, following "0.003%" insert --and 0.0009%; line 66, "mixture" should be wmatrix-w.

Signed and sealed this 11th day of April 1972.

(SEAL) Attest:

EDWARD M.FLETCHER ,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. A weldable ferritic malleable iron casting as set forth in claim 1 wherein said wear layer is covered with a decarburized weldable layer of scattered carbon nodules in a ferritic mixture.
 3. A weldable ferritic malleable iron casting as set forth in claim 2 wherein said decarburized weldable layer is covered by a decarburized intergranular oxidation layer which forms the outside surface layer of said casting.
 4. A weldable ferritic malleable iron casting as set forth in claim 3 wherein said decarburized layers extend below the surface of said casting a distance of between 0.010 and 0.250 inches.
 5. A weldable ferritic malleable iron casting as set forth in claim 2 wherein said wear layer extends below said decarburized layer of scattered carbon nodules in a ferritic matrix a distance of between 0.045 to 0.250 inches
 6. A weldable ferritic malleable iron casting as set forth in claim 1 wherein said decarburized wear layer constitutes the outside surface layer of at least a portion of said casting and has a cross-sectional dimension of between 0.030 and 0.090 inches.
 7. A weldable ferritic malleable iron casting as set forth in cLaim 1 wherein said phosphorus is present in an amount not greater than 0.08 percent.
 8. The method of forming weldable ferritic malleable iron castings having high magnetic permeability and low magnetic remanance and wherein the castings include a body portion of fully ferritic malleable iron covered by a layer of partially decarburized spheroidized martensite in a pearlitic malleable matrix which in turn is covered by a decarburized wear layer of spheroidized carbides in a ferritic matrix, said wear layer being covered by a decarburized weldable layer of scattered carbon nodules in a ferritic matrix and wherein said last named layer is covered by an intergranular oxidation layer forming the outside surface layer of the casting, the method comprising the steps of annealing said castings at a temperature of between 1,800* F. to 1,975* F. in a decarburizing atmosphere containing carbon monoxide, carbon dioxide and hydrogen gas as the active ingredients, said atmosphere being relatively free of oxygen gas, the ratio of carbon monoxide to carbon dioxide being in the range of 1.5 to 1 to 3.5 to 1 by volume, said hydrogen gas being present in an amount of between 15 percent to 30 percent by volume of said decarburizing atmosphere, said annealing being conducted for a period of time sufficient to convert substantially all of the carbides present in said body portion to carbon nodules dispersed in a ferritic matrix, said decarburizing atmosphere being operative to reduce substantially the amount of carbon nodules present in at least said decarburized layers, cooling said castings to a temperature of between 1,400* F. and 1,650 F., and tempering to form spheroidized carbides in a ferritic matrix in said decarburized wear layer.
 9. The method as set forth in claim 8 in which carbon dioxide is present in an amount of 2.1 percent by volume, carbon monoxide is present in an amount of 7.6 percent by volume, hydrogen is present in an amount of 21 percent by volume and nitrogen constitutes the remainder of the decarburizing atmosphere.
 10. The method as set forth in claim 8 wherein said castings are oil quenched following cooling between 1,400* F. to 1,650* F., and said tempering operation includes reheating said castings to between 1,370* F. and 1,430* F., maintaining said castings at said temperature for a period of 1 hour for each inch of cross section casting thickness, cooling said castings at a rate of 10* to 25* per hour to a temperature of between 1,320* F. and 1,280* F., and air cooling said castings to ambient temperature.
 11. The method as set forth in claim 8 wherein said castings are air cooled following cooling to between 1,400* F. and 1,650* F., thereafter reheating said castings to a temperature of between 1,400* F. and 1,650* F. followed by oil quenching of said castings, and said tempering operation including reheating said castings to a temperature of between 1,275* F. and 1,350* F. and holding said castings at said temperature for a period of between 4 and 16 hours followed by cooling in air to ambient temperature.
 12. The method as set forth in claim 10 wherein said step of cooling to 1,320* F. to 1,280* F. includes the step of maintaining said castings at a temperature of between 1,300* F. to 1,320* F. for a period of from 2 to 4 hours. 